1. Field of the Invention
This invention relates to the field of electronics, and in particular to a power amplifier for use in a CMOS device, such as a wireless transmitter.
2. Description of Related Art
CMOS technology is commonly used for devices that require minimal power consumption, such as handheld telephones. Power amplifiers, such as used in the output stage of a wireless transmitter, however, are commonly fabricated with GaAs, Bipolar, or PHEMT technologies. As CMOS technology advances, it is becoming an attractive alternative for use in power amplifiers, based on its small feature sizes, low cost, and relatively high efficiency (low power loss). Because CMOS transistors are small and the cost of fabrication is low, complex designs that may not be economically feasible in other technologies are often quite feasible in CMOS.
FIG. 1 illustrates an example prior art power amplifier 100, commonly referred to as a Class F power amplifier. xe2x80x9cHIGH EFFICIENCY POWER AMPLIFIER FOR MICROWAVE AND MILLIMETER FREQUENCIESxe2x80x9d by William S. Kopp and Sam D. Pritchett, published in the IEEE MTT Symposium Digest, 1989, pp. 857-858, presents the principles of a Class F power amplifier, and is incorporated by reference herein. In a preferred embodiment, the transmission line T is tuned to short even-order harmonics, and a match network Co, Lo, CL is designed to match the load RL, so as to pass the fundamental frequency and attenuate the third-order harmonics. The capacitor CL, being parallel to RL, reduces the impedance associated with the load RL, while Lo cancels any remaining reactive impedance seen by the transistor. The signal is AC coupled through the capacitor Co.
The conventional Class F power amplifier substantially attenuates the even harmonics, via the transmission line T, and the third harmonic via the match network Co, Lo, CL, but the attenuation provided for the third harmonic is often insufficient for some applications. One of the reasons that the third harmonic remains at a high level is that, with a quarter wave transmission line T to cancel the even order harmonics, the impedance of the line T is minimum with respect to the second harmonic, but maximum with respect to the third harmonic. Therefore, third harmonics actually increase at the drain of the transistor M1, and, even with the match-filtering, third-order harmonics at the output of the amplifier can still be high. Also, the precise tuning of the transmission line T to correspond to the second-order harmonics can affect the attenuation that is achievable for the even-order harmonics. Improved attenuation of harmonics properly shapes the output waveform, and also results in higher efficiencies, as less power is wasted propagating unwanted harmonics.
Efficiency enhancement over a wide range of power is essential for today""s wireless applications, because mobile terminals do not usually transmit at maximum output power. A high energy efficiency results in longer battery-life, reduces the requirements for heat dissipation, and so on.
It is an object of this invention to reduce the harmonics produced in a Class F amplifier. It is another object of this invention to increase the efficiency of a Class F amplifier. It is a further object of this invention to provide the reduced harmonics and increased efficiency in a CMOS embodiment of a Class F amplifier. It is a further object of this invention to provide increased efficiency across a wide range of power output from a Class F amplifier.
These objects and others are achieved by providing a CMOS Class F amplifier that uses a differential input to eliminate even-order harmonics, thereby avoiding the need for circuits that are tuned to the second harmonic. This also minimizes the sensitivity of the design to changes in the second harmonic frequency and/or the particular component values selected for the tuned circuit. Third-order harmonics are reduced by controlling the phase relationship between the differential inputs. Additional efficiency is achieved by dynamically controlling the impedance of the amplifier as a function of output power level.